Duchenne muscular dystrophy (DMD) is inherited in a recessive pattern, suggesting that gene therapy could offer an effective treatment if methods can be found to replace the gene in striated muscles throughout the body. Previous attempts to transfer genes to muscles in a systemic manner have been severely limited by an inability to target widely dispersed muscles of adult mammals. For example, anesthesia, invasive surgery, and hazardous co-factors have been required to transduce varying fractions of the cardiomyocyte population efficiently. Similarly, the transfer of genes to the muscles of limbs using plasmids or viral vectors has required either direct injection of individual muscles, or complex surgical procedures performed under anesthesia using hazardous co-factors. Development of an efficient and safe method for systemic gene transfer to muscle could lead to a treatment for a wide variety of muscle diseases including the muscular dystrophies. We have identified a simple and highly efficient method to transfer genes systemically to cardiac and skeletal muscles of adult mammals. This approach utilizes intravenous administration of recombinant adeno-associated viral vectors pseudotyped with the type 6 capsid (rAAV6). Preliminary studies show that rAAV6 vectors can transduce striated muscles throughout the body of adult mice, and that this delivery can be enhanced by co-administration of the vascular permeabilizing agent VEGF. Intravenous injection of rAAV6 vectors expressing micro-dystrophin results in a whole body amelioration of the dystrophic phenotype in adult mdx mice. While these studies represent a clear proof of principle that dystrophin can be delivered to all the striated muscles of an adult mammal, the mechanisms that enable gene transfer to muscle using rAAV6 are unclear, and it will be necessary to increase the efficiency of the approach, address safety issues and apply the methods to larger animal models before they could be tested in the clinic. We will study the mechanisms that enable systemic muscle transduction via the circulation and will conduct a detailed optimization of the protocols. The AAV vector capsids will be modified for improved muscle gene transfer, and we will perform preclinical optimization studies in larger animals. The ultimate goal of this work is to develop a clinically relevant treatment for DMD and other diseases of striated muscle.